It is important in an optical communication system using optical fibers to increase the receiving sensitivity and the frequency utilization efficiency per optical fiber in order to achieve a long-distance and large-capacity communication. It has been proposed to switch to use various modulation schemes that differ in the reachable transmission distance and the frequency utilization efficiency depending on a transmission distance and a transmission capacity that are required because the optical communication has a trade-off relationship between increase in transmission distance and improvement in frequency utilization efficiency. Examples of modulation schemes that differ in the reachable transmission distance and the frequency utilization efficiency include BPSK (binary phase shift keying), QPSK (quadrature phase shift keying), 8QAM (quadrature amplitude modulation), and 16QAM.
One example of optical transmitters that switch to use modulation schemes is described in Patent Literature 1. The related adjustable bit rate optical transmitter described in Patent Literature 1 includes a digital signal processing (DSP) unit and accompanying digital to analog conversion (DAC) circuitry and drives different programmable Mary-Quadrature Amplitude Modulation (M-QAM) schemes. The DSP is programmed to apply a control algorithm and select a proper QAM scheme from the multiple QAM schemes for the signal modulation of the optical transmitter. It is said that the configuration makes it possible to maintain a desired level of transmission performance or optimize the transmission performance without the need for replacing the optical transmitter.
The related adjustable bit rate optical transmitter, however, has the problem that frequency utilization efficiency has a surplus depending on conditions because the granularity is coarse in switching modulation schemes. If a single optical transceiver switches to use a plurality of modulation schemes such as BPSK, QPSK, 8QAM, and 16QAM, it is necessary to implement a digital signal processing circuit having a plurality of algorithms and bit precision that correspond to the plurality of modulation schemes. Consequently, there has been the problem that the power consumption of the optical transmitter and an optical receiver increases, and the control of them becomes complex.
A set-partitioning method in a multi-dimensional signal space is one of the methods by which making the granularity in switching modulation schemes fine down is balanced with preventing the signal processing circuit from getting complex under the above-described trade-off relationship between the transmission distance and the frequency utilization efficiency. The set-partitioning method has gotten attention recently because it has a high affinity for optical signals that are four-dimensional signals intrinsically. For example, Non Patent Literature 1 discloses a technology of obtaining coding gain through multi-dimensional optical coded modulation, that is, SP (set-partitioning)-32-4D (dimensions)-16QAM and SP-128-4D-16QAM.
Non Patent Literature 2 discloses a trellis-coded optical modulation scheme in which convolutional codes are combined with a set-partitioning method, as a method of achieving a transmission distance that exceeds a transmission distance anticipated from the above-mentioned trade-off relationship between a transmission distance and frequency utilization efficiency. The trellis-coded modulation enables a least-square free distance between code sequences to be extended by the convolutional encoding to the square of the distance between signals in the states that are partitioned by the set-partitioning method. This makes it possible to obtain coding gain that surpasses one achieved by the set-partitioning method.